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Old September 1st 06, 01:53 AM posted to rec.radio.amateur.homebrew
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Default MRX-40 modification

Hello OM,
I have build the MRX-40 form a kit many years ago. It
work fine but ...
Now I would like to make a small modification to this receiver.
I would like to replace the tuning capacitor with some kind of diode.
I manage to add a small potentiometer with the shaft getting outside
and I remove the initial capacitor. Now I wonder how and what kind of
diode I could use. I dont have also any idea about what to expect in
the
range of tuning.
May be someone else had the same idea

Any help is will be greatly appraciate.
Happy day
72 de Jean (VE2GHI)


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Old September 2nd 06, 04:52 AM posted to rec.radio.amateur.homebrew
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Default MRX-40 modification

Reposted.
wrote in message
ups.com...
Hello OM,
I have build the MRX-40 form a kit many years ago. It
work fine but ...
Now I would like to make a small modification to this receiver.
I would like to replace the tuning capacitor with some kind of diode.
I manage to add a small potentiometer with the shaft getting outside
and I remove the initial capacitor. Now I wonder how and what kind of
diode I could use. I dont have also any idea about what to expect in
the
range of tuning.
May be someone else had the same idea

Any help is will be greatly appraciate.
Happy day
72 de Jean (VE2GHI)

Try VU2UPX's excellent circuit:
http://www.qsl.net/vu2upx/Projects/dc_rx.htm
This uses varicaps for tuning in a NE602 based receiver (although this is
for 20m).

Also
http://www.hanssummers.com/radio/varicap/index.htm
for G0UPL's testing of several common diodes as varicaps.
Red LED's came out top!
Does anyone else have info on using them as varicaps for tuning?

73s
Klop


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Old September 2nd 06, 11:53 AM posted to rec.radio.amateur.homebrew
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First recorded activity by RadioBanter: Jul 2006
Posts: 85
Default MRX-40 modification

On Sat, 2 Sep 2006 11:52:41 +0800, "Klop"
wrote:


Also
http://www.hanssummers.com/radio/varicap/index.htm
for G0UPL's testing of several common diodes as varicaps.
Red LED's came out top!
Does anyone else have info on using them as varicaps for tuning?


When using a semiconductor in a transparent package, you should
remember that the ambient light will generate a voltage across a
reverse biased junction.

When using a high resistance potentiometer to generate the tuning
voltage, the voltage generated by the ambient light might upset the
voltage across the diode and hence, the capacitance (and resonant
frequency) varies with the ambient light level.

At least keep the ambient light out of any diode with a transparent
package, when used in a high impedance reverse biased circuit.

Paul OH3LWR

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Old September 2nd 06, 06:45 PM posted to rec.radio.amateur.homebrew
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First recorded activity by RadioBanter: Jul 2006
Posts: 464
Default MRX-40 modification

Also
http://www.hanssummers.com/radio/varicap/index.htm
for G0UPL's testing of several common diodes as varicaps.
Red LED's came out top!
Does anyone else have info on using them as varicaps for tuning?


When using a semiconductor in a transparent package, you should
remember that the ambient light will generate a voltage across a
reverse biased junction.

When using a high resistance potentiometer to generate the tuning
voltage, the voltage generated by the ambient light might upset the
voltage across the diode and hence, the capacitance (and resonant
frequency) varies with the ambient light level.


And, if the ambient light was from a mains-powered bulb (fluorescents
would be worse) there would be a significant amount of 50 or 60 Hz
light-flicker energy. This might be sufficient to inject hum into the
signal... flicking light modulates junction voltage, which modulates
junction capacitance, which modulates the RF oscillator frequency.

So, the LED-varicap-tuned receiver might be both drifty, and
hum-prone, if the LED were exposed to light.

At least keep the ambient light out of any diode with a transparent
package, when used in a high impedance reverse biased circuit.


Yup. A dab of black paint would be a good idea.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
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Old September 7th 06, 08:20 PM posted to rec.radio.amateur.homebrew
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First recorded activity by RadioBanter: Jun 2006
Posts: 1,374
Default MRX-40 modification

Klop wrote:
. . .
Also
http://www.hanssummers.com/radio/varicap/index.htm
for G0UPL's testing of several common diodes as varicaps.
Red LED's came out top!
Does anyone else have info on using them as varicaps for tuning?


It's too bad he didn't think of trying zener diodes. By using zeners,
you can choose a wide range of capacitances -- low voltage zeners have
the most C and higher voltage zeners have less, high power zeners more
than low power ones. I've used them for decades for such things as RIT
adjustment. A reverse-biased transistor emitter-base junction is also
basically a zener, with typical breakdown voltage of around 5 volts, so
this works fine as a varicap when a fairly large amount of C is needed.

Any reverse biased diode will behave like a varicap, although as G0UPL
found out, small signal diodes (intentionally) have a very small amount
of C. The diodes sold as varicaps are doped to provide a particular and
repeatable C-vs-V characteristic, but otherwise are the same as other
diodes.

The real problem with using varicaps of any kind for tuning is the
relatively high and nonlinear temperature coefficient.

Roy Lewallen, W7EL


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